This invention relates to a method for laser cutting wherein nitrogen is used to avoid oxidation of the metal during passage of the laser beam.
Laser cutting is state of the art for cutting of thick or thin metal parts according to most diverse geometries. As sublimation of the metal occurs at various temperatures over and under the part being cut, the laser beam is confined by means of pure nitrogen which acts as a cutting gas to prevent fusion of the metal which would allow alloy of air molecules to the metal.
Pure nitrogen is obtained by a cryogenic process and supplied in liquid form. The liquid nitrogen is therefore heated to be brought in gaseous form to feed the laser cutting equipment at a pressure about 25 bar and at a rate of 50 m3/hour or a multiple thereof depending on the number of laser beams of the cutting machine.
The use of pure nitrogen in liquid form is very expensive due to the cryogenic process. Handling thereof is also expensive because it requires heavy containers to withstand a pressure of approximately 30 bar during storage and transit. Furthermore, heater equipment is needed to heat the liquid nitrogen to bring it in gaseous form to feed the cutting machine. All these operations require precautions and safety measures in terms of personnel and equipment and they are energy consuming, which further raises the cost of laser cutting.
In the mid-1980""s, based on the varying rates of permeation of air""s component gases, a hollow fiber membrane technology was developed to generate technical nitrogen, i.e., gaseous nitrogen up to about 99,9% pure. According to this technology, compressed air passes through a high density bundle of hollow membrane fibers whereby the relatively fast gases present in the airxe2x80x94oxygen, carbon dioxide and water vaporxe2x80x94diffuse through the semi-permeable membrane surface, leaving behind a stream of high purity gaseous nitrogen. The operational performance of the system is determined by a number of factors. For example, the flow rate of nitrogen produced by the membrane fibers varies inversely with the nitrogen purity required. The nitrogen flow rate is directly related to the air supply pressure. Further, the nitrogen flow increases as the temperature of the feed air increases. The membrane fibers are engineered into modules available in an array of sizes and capacities, and the system is an advantageous source of nitrogen when liquid nitrogen supplies are impractical, unreliable and cost-prohibitive.
U.S. Pat. No. 5,763,855, the content of which is incorporated herein by reference thereto, discloses a method for laser cutting wherein nitrogen is used to avoid oxidation of the metal during passage of the laser beam, comprising the steps of providing an atmospheric air supply; compressing said atmospheric air; driving said compressed atmospheric air through a first density bundle of hollow membrane fibers to permeate oxygen and air""s component gases through said hollow membrane fibers and yield technical nitrogen at a first purity at the end of said first high density bundle of hollow membrane fibers; and collecting said technical nitrogen at the end of said first high density bundle of hollow membrane fibers.
U.S. Pat. No. 5,383,957, the content of which is incorporated herein by reference thereto, discloses a multistage membrane system for the production of high purity gases.
What is needed therefore is a means to improve the laser cutting by a method which avoids the drawbacks resulting from use of nitrogen in liquid form obtained by the cryogenic process by using technical nitrogen obtained via the aforesaid hollow fiber membrane technology in an efficient and cost effective manner.
The method provides technical nitrogen instead of pure nitrogen in liquid form which has to be heated to be brought in gaseous form for laser cutting. Atmospheric air is compressed and driven through a cascade of high density bundles of hollow membrane fibers to permeate oxygen and air""s component gases and yield technical nitrogen, and the pressure of said technical nitrogen is boosted to feed the laser cutting equipment. The method thus improves the laser cutting so as to avoid the drawbacks resulting from use of nitrogen in liquid form obtained by the cryogenic process by using technical nitrogen obtained via the aforesaid hollow fiber membrane technology in an efficient and cost effective manner.